1. Field of the Invention
The present invention relates to a cut-off frequency setting circuit of a filter circuit, and more particularly to a cut-off frequency setting circuit of a filter circuit composed of an IC.
2. Description of the Related Art
FIG. 1 shows a conventional cut-off frequency setting circuit. In this prior art, in order to set the cut-off frequency of a primary low-pass filter circuit 1 composed of a pair of differential transistors Q3, Q4, a transistor Q2 for constant-current source, transistors Q5, Q6 connected for composing a current mirror, an emitter follower Q7, capacitance C1 and others, an analog current is supplied to the base and collector of the transistor Q1 for forming the current mirror circuit 2 together with the transistor Q2 for providing a constant-current source from the outside through an analog input terminal 5 for adjustment of cut-off frequency, thereby setting the cut-off frequency of the filter circuit 1.
An analog current generating circuit 3 externally attached to the outside is composed of, for example, resistances R1, R2 connected between a supply voltage +B and the ground. In FIG. 1, numeral 4 denotes a signal input terminal for feeding in the signal to be filtered in the filter circuit 1, and Vcc is a power source in an IC (Integrated Circuit) 18.
In the conventional circuit, however, when one attempts to change the cut-off frequency of the filter circuit 1, external parts (resistances R1 and R2 in the case of FIG. 1) must be exchanged. Accordingly, in the prior art, not only the external circuit needed, but also a space must be kept for the external circuit when mounting on a circuit board, which makes high density mounting difficult.
Besides, by fluctuations of the supply voltage +B, the analog current varies, and thereby the input current and output current of the current mirror circuit 2 change. Fluctuations of the output current of the current mirror circuit 2 cause fluctuations of constant-current of the filter circuit 1, which leads to variations of dynamic resistances of the transistors Q3, Q4, and thereby the cut-off frequency of the filter circuit 1 changes.